The present invention relates to a method and an apparatus for controlling a traveling wave motor as disclosed in DE 3,306,755.A1. The vibrating stator of such a motor includes ultrasound excitation systems which are equipped with electrodes and are connected in a friction lock with the actual vibrating body. At a surface region of the vibrating body, the vibrating stator is axially pressed into contact with a rotor. Rotational rotor movements are generated by mechanical waves that are excited in the excitation system, with the individual vibrating surface elements of the vibrating body moving on an approximately elliptical path so that, due to the friction lock between the stator and the rotor surface, tangential forces are generated in the contact zone between the two surfaces and lead to the formation of a rotary movement and a torque on the rotor.
The operating state of such a traveling wave motor is dependent on a plurality of parameters, particularly the temperature, the axial pressure between stator and rotor and the frequency, amplitude and shape of the excited traveling waves. The setting of the desired operating parameters, adaptation of the motor feeding frequency if there is a change in the natural mechanical frequency due to temperature and long-term influences, and the required limitation of the amplitude of the traveling waves in order to protect the excitation system and the contact zone between the rotor and the vibrating stator, require such motors to be guided by one or several control circuits. For this purpose, information about the actual state of the stator-rotor system, particularly the frequency, amplitude and shape of the traveling wave, must be supplied to the control system.
DE 3,634,329 already discloses an actuating circuit for an ultrasonic wave motor in which it is ensured that the motor is always actuated at its resonant frequency. For this purpose, a monitoring electrode is provided to detect the phase difference between the signal of the monitoring electrode and a periodic actuation voltage applied to drive the motor.
A drawback of this and is that only indirect information about the presence and characteristics of the traveling waves is evaluated since only one sensor surface is available and therefore only the amplitude of one surface region can be sensed. Amplitude values and phase relationships of the deflections of different surface regions of the vibrating body are not detected by the prior art actuation circuit. In particular, a standing wave, which does not directly produce any effects on the motor, cannot be distinguished from a traveling wave since both create the same signal image on a single-surface sensor.